You need to adjust the contrast. To do this, switch on the unit, press and hold down the Back Light Key. You will then see the following:
Using the up and down blue cursor keys, you can then adjust the contrast. When you have finished, press the Back Light Key again.
In cold conditions the contrast is affected, so the DSP-100 contains a thermistor which adjusts the contrast of the LCD. If the DSP is cold and you switch it on, you may see a dark screen. Within 2 seconds it will adjust itself automatically to a suitable contrast.